Method for casting steel ingots

ABSTRACT

In casting steel ingots, e.g., for rolling purposes, the ingot mould is filled with effervescent steel in a single casting operation, after a time the liquid contents are killed by addition of a deoxidizing agent, and thereafter a gas, preferably air, is blown through the molten steel from a lance introduced into the molten steel from above and into proximity to the bottom of the mould. The lance preferably consists of a non-heatinsulating tube of a normal and usual steel composition, and heat discharge from the upper part of the molten steel is preferably decreased by applying an insulating layer locally onto the inner wall of the ingot mould and/or strewing a heat insulating powder onto the steel.

United States Patent [191 Van Goudoever et al.

[451 Sept. 25, 1973 1 METHOD FOR CASTING STEEL INGOTS [75] Inventors: Sebastian Van Goudoever,

Santpoort; Leonard Jansse, Ijmuiden, both of Netherlands [73] Assignee: Koninklijke Nederlandsche Hoogovens En Staalfabrieken N.V., ljmuiden, Netherlands [22] Filed: Aug. 12, 1971 [21] Appl. No.: 171,339

[30] Foreign Application Priority Data Aug. 14, 1970 Netherlands 7012060 [52] US. Cl 164/55, 75/51, 75/59 [51] Int. Cl B22d 27/18, B22d 27/20 [58] Field of Search 164/55, 57,56; 75/51 [56] References Cited UNITED STATES PATENTS 2,882,571 4/1959 Easton 75/58 3,208,117 9/1965 Goedecke et al... 2,837,800 6/1958 Hachiya et al. 164/56 FOREIGN PATENTS OR APPLICATIONS Primary ExaminerJ. Spencer Overholser Assistant ExaminerV. K. Rising Att0meyJ. Y, Houghton 5 7 ABSTRACT In casting steel ingots, e.g., for rolling purposes, the ingot mould is filled with effervescent steel in a single casting operation, after a time the liquid contents are killed by addition of a deoxidizing agent, and thereafter a gas, preferably air, is blown through the molten steel from a lance introduced into the molten steel from above and into proximity to the bottom of the mould. The lance preferably consists of a non-heat-insulating tube of a normal and usual steel composition, and heat discharge from the upper part of the molten steel is preferably decreased by applying an insulating layer 10- cally onto the inner wall of the ingot mould and/or strewing a heat insulating powder onto the steel.

4 Claims, 2 Drawing Figures AWAY A This invention relates to a method for casting steel ingots, in particular for rolling purposes, in which socalled effervescent steel is cast into an ingot mould and in which the liquid contents of the mould after some time are killed by the addition of a deoxidizing agent.

This method, which is often indicated in many countries by the English expression stabilized rimmer, was chosen in practice for avoiding several disadvantages related to the nature and characteristics of ingots for rolling consisting of killed steel or of effervescent(rimmed) steel, and related to the casting thereof.

Steel ingots made from killed steel are very well suited for rolling sheet steel therefrom, which is not much liable to ageing. Thus sheets therefrom are, even after long storing periods, still suited for deep drawing operations.

However, it is difficult to avoid, by the calm nature of the solidification process, that oxides are segregated in the proximity of the outer wall of the ingot, so that sheets rolled from such ingots usually have a surface, which does not have good characteristics as to purity, smoothness and appearance.

When casting effervescent steel to ingots-the skin of the solidifying ingots is continuously swept clean by the turbulent steel, so that sheets rolled from such ingots have a particularly clean surface of attractive appearance. A disadvantage of ingots thus cast consists in that sheets rolled therefrom are liable to ageing in view of nitrogen present therein in solution and not chemically bound. Thus the possibilities to use such sheets for deep drawing purposes are limited.

The known method as given in the preamble aims at combining the resistance to ageing ofingots from killed steel with the clean, smooth and beautiful surface of ingots cast from effervescent steel while avoiding the disadvantages inherent to both types of steel.

In this known method the ingot mould is first provided with a so-called hot top at the inner side close to its upper edge, said hot top consisting of thermally insulating material. Such hot tops are very well known and available in the trade in different types and compositions.

Thereupon the ingot mould is filled with effervescent steel up to the lower edge of the hot top. After some time, usually after about to 6 minutes, during which a clean solidified skin has formed in contact with the wall of the ingot mould, a deoxidizing agent is introduced into the liquid metal in the mould, which kills this remaining liquid metal in the ingot mould. A usual deoxidizing agent in this respect is aluminium.

After finishing of the reaction with the deoxidizing agent the ingot mould is further filled with steel up to its upper edge, after which the bath is covered by a powdery substance restricting the heat loss in an upward direction.

An advantage of this method is that an ingot is obtainedwith a clean skin, giving rolled sheets with clean and smooth surfaces. Moreover, it is possible to make sheets from such an ingot by rolling, which are very well resistant to ageing. Nevertheless this method also has disadvantages. One of these is that it is necessary to heat the bath in the ladle (of e.g., 100 tons) to a temperature which is about 30C higher than usual, because it is necessary to apply additional filling of the ingot mould after the deoxidizing reaction. Such a higher temperature of tapping the ladle and filling the ingot mould has several unfavourable consequences. In general a higher temperature when tapping means that the control of the gas development and thus of the flow of gas in the steel during the solidification of the skin becomes more difficult. Moreover, the wear of the casting plates, on which the ingot moulds are positioned, and of the ingot moulds themselves, increases rapidly with a higher temperature of casting. Wear in this respect means among others the attack as a result of erosion and corrosion of casting plates and ingot moulds by the falling jet of liquid steel, and the mechanical wear caused by the stripping of the ingot moulds from the ingots, for which stripping very high forces are necessary because as a result among others of the high casting temperature the ingots stick intimately to the casting plates and/or the ingot moulds.

A higher wear of plates and ingot moulds results in a more rapid depreciation thereof and thus a higher price of the steel. Another disadvantage of said known method is formed by the longer time periods necessary for filling of the ingot moulds. This is so because by the filling in two phases the total filling time of the ingot mould in the average becomes longer, and thus the casting crane is less available for other purposes or further use and the same is true for the casting ladies, the ingot moulds and the casting plates. This also disadvantageously influences the price of the steel.

The present invention aims at removing these disadvantages and in particular to cast ingots of qualities as given above by means of a single filling operation of the ingot moulds with molten steel of a lower temperature, while maintaining an effervescent solidification during 5 to 6 minutes after filling of the ingot mould. In this respect the invention consists in that in a method as given in the preamble the ingot mould is filled with molten steel in one casting operation and that after adding the deoxidizing agent the still molten part of the contents of the ingot mould are homogenized by means of the blowing of a gas through the molten steel from a lance introduced into this steel from above to the proximity of the lower end of the ingot mould.

It is remarked that it has been tried before to manufacture a stabilized rimmer ingot with a skin of sufficient thickness, solidified effervescently, by means of a single filling operation of the ingot mould, but up to now this has always been a failure. It always appeared necessary to apply supplemental filling of the ingot mould in order to improve the mixing and distribution of the deoxidizing agent.

In particular the applicatn has tried before to initiate a development of gas in the steel after the killing thereof, by introducing blocks of solid material into the bottom part of the molten bath in the ingot mould, which blocks in view of the temperature in said zone will be decomposed while giving a development of gas. Such trials have always been a failure up to now. On the other hand it has also appeared to be very difficult, disadvantageous or even impossible from a technological view-point to have gas introduced into and bubbling through the bath by introduction through the wall of the ingot mould or through the wall of the casting plate. This is so because at the opening of the passages for the gas in mould or plate, necessary thereto, the steel will solidify rapidly and will thus close such openings.

Due to the new method proposed according to the present invention it is possible to fill the ingot moulds in one cast with steel of a temperature being about 30C lower than otherwise necessary.

It is preferred in the method according to the invention to decrease the heat discharge from the upper part of the bath by applying a hot top and/or by strewing heat insulating powder onto the bath, which as stated before is known as such and is also preferred in the above-given known method. Several substances suitable for such purposes are available in the trade.

As a gas to be blown through the bath first of all an inert gas such as argon is suited. However, the high costs thereof make this less attractive. Surprisingly it has now appeared that corresponding results are obtainable by blowing air through the bath. Most probably the disadvantage, which this was expected to have, that nitrogen and oxygen from this air would disadvantageously affect the composition of the steel is negligible in practice. On the other hand it has appeared that the final results of the new method are dependent upon the circumstances, under which such air is blown through the bath.

The blowing of such air is possible with the aid of a lance, which is cooled and/or heat-insulated. Thereby a structure of this lance is possible, whichduring a short period is resistant against the high temperature of the steel bath. However, a disadvantage of such a structure is formed by the danger that part of the heat insulation is loosened from the lance and gets into the bath. This could cause serious pollutions of the bath. More dangerous even is the possibility that by falling off of part of the heat insulation the cooling jacket of the lance is opened by melting away of its wall,,so that the cooling fluid enters the ingot mould. Particularly if such a fluid is a liquid, e.g., water, an explosion could be the result thereof.

A surprising solution for this problem was found in completing the present invention by using a blowing lance, consisting of a single non-insulated tube of a usual steel quality. The result thereof is that after the blowing period of short duration the steel tube melts away and the steel thereof is solved into the steel bath in the ingotmould. As the tube with respect to the charge in the ingot mould only has a negligible volume, the metallurgical composition of this tube is not very much critical in influencingthe composition of the ingot. Thus the requirement that the tube should consist of a normal and usual steel quality is sufficient, with sufficient freedom as to the exact composition.

Due to the blowing of the air through the bath the deoxidizing agent mixes rather rapidly and completely through the bath of steel in the ingot mould. Nevertheless it has appeared that the manner of feeding of this deoxidizing agent to the bath is able to influence the final quality of the ingot. Thus it has appeared that the best results were according to the invention obtained by using a deoxidizing agent, preferably aluminium, in the shape of a thick ribbon introduced vertically from above into the bath. It is easily possible to do this in such a way that the aluminium ribbon will move approximately to the bottom of the bath in the mould. Thereby part of the aluminium, after being molten, is intimately mixed with the still liquid part of the steel, as long as there is still development of gas and thus a flow in the bath. A further homogenizing is thereafter obtained by the blowing air through the bath.

For sake of completeness it is remarked that several other methods are known, which will be described below.

In French Pat. specification No. 1,386,390 the manufacturing of stabilized rimmer ingots has been described, in which aluminium is introduced as a powder into the steel bath in the ingot mould. This powder is conveyed into the bath with the aid of a supporting gas through a blowing lance. Thus the gas supply should take place simultaneously with the supply of the powder and should be so abundant that the aluminium powder is entrained by the gas entirely through the lance without sintering or even melting in the lower part of this lance by the high temperature prevailing in this area. A quantity of 10 liters of gas at atmospheric pressure and temperature (standard liters) per ton of steel is mentioned. it is clear that such a high quantity of gas cannot be allowed to be chemically reactive with the steel, as otherwise it would considerably influence the composition thereof. Thus in this method argon should be used as this gas. As an additional advantage of such injection of gas the scavenging of impurities is mentioned, for which reason it is even recommended to begin the introduction of gas before the beginning of the aluminium injection.

According to the present invention aluminium or another deoxidizing agent is first introduced into the bath and only thereafter the gas is injected. This gas does not serve the purpose of scavenging the steel, but of homogenizing it as a result of a stirring action. For this purpose a considerably lower quantity of gas is necessary (in the order of magnitude of 1.5 standard liters per ton of steel) and by this much lower quantity this gas needs not be argon or another inert and expensive gas, but it could be air which is of course much cheaper. According to this method of the invention it has not appeared necessary to scavenge the molten steel by the gas.

In a later proposal of the same inventor, known from French Pat. specification No. 1,405,108, the scavenging with gas is in essence abandoned. Aluminium powder is introduced mechanically into the lower part of the. molten steel bath, after which it is first allowed to melt in this area before it is metered to the steel. This metering may take place by expelling the molten aluminium from a bell-shaped space with the aid of a gas pressure above the aluminium. In essence no or not much gas has to be introduced into the molten bath of steel. No gas is used to homogenize the bath after the addition of the aluminium by a stirring action, contrary to what is proposed by the present invention.

In U.S. Pat. No. 3,208,117 a method has been described for scavenging molten steel within a mould with the aid of a gas in order to avoid the formation of faults and defects of the casting in the core of the castings. Contrary thereto the present invention aims at homogenizing the steel bath after the formation of the solidifled skin and after killing of the remainder of the molten steel. This entrains the scavenging away mainly of impurities closely below or inside the ingot outer surface. For the method given in this U.S. patent a quantity of gas is necessary, which is ten to fifty times the quantity of gas necessary for the present invention, which makes this known method considerably more expensive than the method according to the invention. During a large part of the gas supply period it is possible according to this U.S. patent to have aluminium powder be entrained by the gas. This powder will then, however, not so much have a function as a deoxidizing agent, but more as a gas forming substance, by which the scavenging by means of the gas is improved.

The invention will now be explained in more quantitative detail by the following example having reference to the accompanying diagrammatic drawings, wherein:

FIG. 1 represents a vertical section through a filled ingot mould into which deoxidizing agent is being introduced, and

FIG. 2 represents the subsequent introduction of air thereinto by way of a fusible steel lance.

EXAMPLE Referring to the drawings, in this example an ingot mould l for 20 tons of steel is provided with a usual hot top 2, so that after the casting the hot top volume is about 13 percent of the ingot volume. The hot top consists of plates, which are mutually connected and secured with angle parts and which are secured to the ingot mould by several nails 3. This is a usual method when manufacturing so-called ladle killed steel. The mould is positioned onto a casting plate 4 with its widest end of its somewhat tapering configuration in contact with the plate (big end down"). Steel with a composition of 0.062 percent of carbon, 0.310 percent of manganese, 0.008 percent of phosphorus, 0.016 percent of sulphur and a quantity of copper, tin, chromium and nickel of 0.096 percent is cast into the ingot mould at a temperature of 1 ,5 65C up to almost the upper end of the hot top plates, while adding 50 grams of NaF and 50 grams of fine aluminium grains per ton of steel. After the steel has been allowed to skin during about 6 minutes an aluminium ribbon 5 is shot downwards into the bath vertically from above in the centre of the mould at a speed of 5.8 in per second, said ribbon having transverse dimensions of 5 by 25 mm. Immediately thereafter a steel pipe 6 of an outer diameter of 14 mm and an inner diameter of 7 mm is submerged in the liquid central part of the bath in the ingot mould, while dry air at 3,500 standard liters per hour is blown therethrough. Thereafter air is blown through the tube for a further 28 seconds at 5% atmospheres gauge pressure. During this blowing a noticeable turbulence occurs in the steel, by which the aluminium present is uniformly distributed through the bath.

The remaining part of the steel pipe is thereupon withdrawn from the bath and the steel bath is covered with a usual covering powder for counteracting the discharge of heat at the top surface. Thereafter the ingots are treated in a manner usual for so-called killed ingots.

It appears that from the ingots thus obtained it is possible to obtain sheets by rolling, which have a very high quality as to deep drawing characteristics, which maintain a beautiful and uniform surface after deep drawing and which are highly resistant against ageing.

We claim:

1. A method for casting steel ingots, in particular for rolling purposes in which so called effervescent steel is cast into an ingot mould and in which the liquid contents of the mould after some rimming has taken place are killed by the addition of a deoxidizing agent, characterized in that the ingot mould is filled with molten steel in one single casting operation, and that after adding the deoxidizing agent the remaining molten part of the contents of the ingot mould are homogenized by means of the blowing of a gas through the molten steel from a lance which consists of a non-heat-insulated tube of a normal and usual steel and which is introduced into the molten steel from above and into proximity with the lower end of the ingot mould, the quantity of gas introduced being sufficiently to effect a homogenizing action and being of the order of about 1.5 standard liters per ton of the steel.

2. A method according to claim 1,

characterized in that the heat discharge from the upper part of the molten steel in the ingot mould is decreased in a manner known per se by applying an insulating layer locally onto the inner wall of the ingot mould.

3. A method according to claim 1,

characterized in that the heat discharge from the upper part of the molten steel in the ingot mould is decreased in a manner known per se by strewing a heat insulating powder onto said steel.

4. A method for casting steel ingots, in particular for rolling purposes, in which so-called effervescent steel is cast into an ingot mould and in which the liquid contents of the mould after some time are killed by the addition of a deoxidizing agent, characterized in that the ingot mould is filled with molten steel in one casting operation and that after adding the deoxidizing agent the still molten part of the contents of the ingot mould are homogenized by means of the blowing of air through the molten steel from a lance introduced into this steel from above to the proximity of the lower end of the ingot mould. 

2. A method according to claim 1, characterized in that the heat discharge from the upper part of the molten steel in the ingot mould is decreased in a manner known per se by applying an insulating layer locally onto the inner wall of the ingot mould.
 3. A method according to claim 1, characterized in that the heat discharge from the upper part of the molten steel in the ingot mould is decreased in a manner known per se by strewing a heat insulating powder onto said steel.
 4. A method for casting steel ingots, in particular for rolling purposes, in which so-called effervescent steel is cast into an ingot mould and in which the liquid contents of the mould after some time are killed by the addition of a deoxidizing agent, characterized in that the ingot mould is filled with molten steel in one casting operation and that after adding the deoxidizing agent the still molten part of the contents of the ingot mould are homogenized by means of the blowing of air through the molten steel from a lance introduced into this steel from above to the proximity of the lower end of the ingot mould. 